The whole construction uses mitered joints. To make those mitered joints I ripped the bottom and sides at 45º on a table saw and also cut the sides sides at 45º saw before gluing.
As for the rounded corners, this is just sanding with an orbital sander with 80 or 150grit to smooth over after gluing. It’s really a way to hide my slightly mis-measured miter cuts.
Finish procedure is 150grit sanding, coat of wipe on poly, 220grit sanding, coat of wipe on poly, 220grit sanding, final coat of wipe on poly.
Here are some other images which should help clarify somewhat:
I suppose that I should note that cutting miters on a table saw is a somewhat intermediate to advanced table saw cut as ideally you run a second spoil board against the fence so that you can make percice cuts with out the miter edge sneaking underneath the fence.